Herein, we introduce a reusable Brønsted
acidic ionic liquid
gel (BAIL gel) obtained by treating 1-methyl-3-(4-sulfobutyl)-1
H
-imidazolium hydrogen sulfate with tetraethyl orthosilicate
(TEOS). The grafting of the Brønsted acidic ionic liquid to the
surface of TEOS has increased the catalytic activity of the material
and also simplified catalyst recovery from the reaction mixture. This
reaction has a wide substrate scope, and the BAIL gel represents a
new catalyst for the synthesis of benzoxazoles, benzimidazoles, and
benzothiazoles. The method shows attractive characteristics such as
high yields, recyclable catalyst, and work-up simplicity. More importantly,
no additional additives or volatile organic solvent and inert atmosphere
are required for the reaction, and the BAIL gel has shown a great
promise for industrial applications. To the best of our knowledge,
the synthesis of benzoxazoles, benzimidazoles, and benzothiazoles
using a recyclable heterogeneous ionic liquid gel was not previously
reported in the literature.
A highly efficient and green strategy for the synthesis of 2-arylbenzoxazoles, 2-arylbenzimidazoles, and 2-arylbenzothiazoles catalyzed by phosphonium acidic ionic liquid has been developed via the condensation of o-aminophenol, o-phenylenediamines, and o-aminothiophenol, respectively, with aldehydes. The reaction has a good yield, the broad substrate scope, and mild condition. Triphenyl(butyl-3-sulphonyl) phosphonium toluenesulfonate catalyst was easily obtained from cheap and available starting materials through a one-pot synthesis. Its structure was identified by 1 H NMR, 13 C NMR, 31 P NMR, and FT-IR techniques. Other properties including thermal stability and acidity were determined by TGA and Hammett acidity function method. Interestingly, the catalyst can maintain its constantly outstanding performance till the fourth recovery.
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